The Rational Design of Low‐Barrier Fluorinated Aluminum Substrates for Anode‑Free Sodium Metal Battery

Abstract Anode‐free Na metal batteries are acclaimed for their high energy densities achieved through current collectors in situ plated with Na metal in the absence of active negative materials. The advancement of these devices hinges on the development of affordable current collectors for effective Na deposition and the design of advanced electrolytes with suppressed Na metal loss as measured against the poor cycling performance and safety issues associated with traditional organic electrolytes and convectional current collectors. Herein, the authors report a novel strategy for fabricating an Al current collector through annealing and fluorination using hydrofluoric acid to optimize its crystal orientation and surface properties in a bid to establish highly reversible Na deposition/dissolution processes. Through a series of characterization, electrochemical, and computational analyses, it is ascertained that the F‐rich surface of the (100)‐oriented Al substrate provides high‐affinity nucleation sites that initiate and sustain a uniform Na metal deposition and form a propitious solid electrolyte interphase layer. The present anode‐free Na metal battery prototype comprised of the treated Al substrate, a high mass loading Na 3 V 2 (PO 4 ) 3 positive electrode, and an ionic liquid electrolyte achieves a high average Coulombic efficiency (≈98%) over 50 cycles: indeed, groundbreaking performance for uncoated current collectors that do not require activation cycles.